International patent application no. WO 98/21818 discloses a system for tracking the resonant frequency of an electrically resonant structure in which a variable frequency oscillator, which generates an excitation signal of a variable frequency encompassing the possible resonant frequency range of the target resonant structure, is connected to the resonant structure by a bi-directional RF transmission line. The proportion of the excitation signal energy reflected and the proportion dissipated by the resonant structure will depend upon the difference between the frequency of the excitation signal and the resonant frequency of the resonant structure, and the transmission line incorporates a directional coupler, which generates a directional coupler signal proportional to the reflected signal from the resonant structure. The directional coupler signal is conditioned by a processor to provide a feedback signal to the input of the variable frequency oscillator such that the mean frequency of the excitation signal is caused continuously to track the varying resonant frequency of the resonant structure.
This arrangement has particular application in the non-contact torque measurement using SAW (surface acoustic wave) devices as the sensing elements. Many such applications, however, use two SAW devices attached to a rotating shaft in such a way that when torque is applied one resonator is put in tension whilst the second is put in compression. This causes the resonant frequency of the first device to reduce whilst the second will increase. The two devices would normally have a nominal difference between them of 1 MHz, such that with torque the output from the system is a difference frequency that changes about 1 MHz with applied torque. However, in order to be used in conjunction with the tracking system of the prior art, the two sensors on the shaft must be electrically connected to the stator of the assembly via two pairs of non-contacting rotary coupled transmission lines. The use of two pairs of couplers has the disadvantage that the size and complexity of the mechanical assembly is increased, and thereby the cost. In addition, the rotary coupled transmission line can load the SAW resonator and thereby modify its frequency. As the system is a differential one, if both couples modify their respective sensor response by the same amount, then this effect can be cancelled out, but if the two channels are not identical then an error in the reading obtained can result.
The arrangement of the prior art has the further disadvantage that the requirement for a directional coupler increases the complexity of the arrangement.